While liquid crystal displays are ubiquitous in dozens of commercial and industrial products, they have long been plagued with a limitation: If not viewed within a minimal angle from perpendicular to the screen, the image suffers a sharp drop-off in brightness, to the point that nothing can be seen. Backlighting systems have helped, but now a polymeric display technology may eliminate the problem entirely.

Developed in a joint venture by Dow Chemical Company, Midland, MI, and Delta Optoelectronics, Inc., Hsin-chu, Taiwan, the polymeric light-emitting display (pLED) system makes use of Dow's green polyfluorene copolymer film. The ultrathin (70 to 100 nm) two-layer material can emit more than 10 times the brightness of a CRT television screen, as high as 16 lumens per watt, on just 5 volts. They exhibit very low turn-on voltages (green <2.5V), making them well-suited for battery-powered devices. Image quality is said to be high contrast and high resolution, with switching speeds up to 1,000 times faster than ordinary LCD screens.

The Dow polyfluorene films are very stable in light and air, maintaining their appearance, mechanical integrity and luminescence properties for years. They can be constructed using a variety of cathode materials including calcium, aluminum and magnesium alloys, and are useful in combination with a variety of hole transport layer materials including PEDOT/PSS (Baytron-P from Bayer), polyanilines and some fluorene copolymers. Polyfluorenes are solution-processable by thin-film coating techniques such as spin coating, ink jetting and other printing methods. They are very soluble in a variety of common organic solvents including toluene, xylene, mesitylene and methylene chloride.

Green is the first of the emitting polymers to be produced on a commercial scale. Dow will launch a red pLED material early this year, to be followed by blue and yellow. The low cost of the pLED technology is anticipated to significantly improve the market for flat panel displays.

--RM

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